This invention relates to bending glass sheets by the gravity sag bending technique, and to the adaptation of that technique to the production of a shape having a sharply bent line extending a part of the way across the sheet. More particularly, this invention deals with the production of a shape having a line of sharp bending which is considerably more sharply bent at one end than at the opposite end. Such a shape is of interest to automobile stylists to match the contours of an automobile body. As an illustration, such a shape may call for the radius of curvature at one end of the line of bending to be as small as about 0.25 inch (6.4 millimeters) to about 4 inches (10 centimeters) while the other end of the bend may have a gentle radius of curvature on the order of 75 inches (1.9 meters) to about 200 inches (5 meters) or greater.
Various techniques have been developed to apply intensified heat along a line extending across a glass sheet so as to enable that portion of the glass sheet to be bent more sharply than the rest of the glass sheet, thereby producing a so-called "V" bend in the glass. These techniques most commonly include the application of electrical resistance heating through a wire ribbon held slightly above or in contact with the glass surface, or through an electroconductive strip applied temporarily or permanently to the glass surface. However, the only prior art of which we are aware that deals with a "V" bend which is sharper at one end than at the other end is U.S. Pat. No. 3,880,636 to M. W. Tobin and T. J. Reese. In that patent, a ribbon of electroconductive material whose resistance varies along its length is sandwiched between two plies of glass being bent simultaneously for subsequent lamination. While that arrangement is capable of producing tapered "V" bends, it would be desirable if certain aspects of that process could be improved. Placing the ribbon between the glass plies tends to create a slight gap between the plies, making it more difficult to subsequently form a good lamination. using a metallic ribbon between the plies can mar the glass, but graphite ribbons are fragile and thus difficult to handle. Moreover, a graphite ribbon is consumed during each bending cycle and must be replaced each time. It would also be desirable to reduce the amount of distortion produced in the glass in the vicinity of the wide end of a tapered ribbon.
U.S. Pat. Nos. 3,865,680 to Reese et al. and 4,002,450 to Hamilton et al. both show production of "V" bends in glass sheets using electroconductive ribbons having areas of varying resistivity. Neither of these patents, however, deal with "V" bends having widely differing radii from one end to the other.
Copending patent applications related in part to the present invention include U.S. patent application Ser. No. 725,054 filed Sept. 21, 1976 by T. J. Reese et al. and U.S. patent application Ser. No. 714,067 filed Aug. 13, 1976 by T. J. Reese. Neither application deals with tapered "V" bends.